Metal easy open can lid superior in can openability and process for production of thereof

ABSTRACT

A metal easy open can lid, and a process for producing the same, wherein the can openability is improved and the occurrence of defects in the resin film at the time of shaping is eliminated whereby repair coating of the inner and outer surfaces are made unnecessary by making the cross-sectional shape of the opening guide groove of the easy open can lid an S-shape are provided, wherein, when forming the opening piece for easy manual opening in the metal lid, the properties of the resin film on the metal sheet, the shape and dimensions of the shoulder portions of the punch and die, the clearance, the residual thickness of the opening guide groove, the degree of the push-back processing, etc. are specified so as to achieve a superior can openability without breakage of the resin film by forming an opening guide groove with an S-shaped cross-section.

TECHNICAL FIELD

The present invention relates to a metal easy open can lid which can beused for beverage cans and other general food cans and a wide range ofother applications and to a process for the production thereof, morespecifically it relates to an easy open can lid, composed of a surfacetreated metal sheet consisting of a metal sheet such as a sheet steel,an aluminum sheet, or metal sheet which has been plated with tin orgiven surface treatment coating such as a chromate coating, paint, andresin laminate, provided with an opening guide capable of easy openingof part of the lid manually.

BACKGROUND ART

In the past, easy open can lids enabling manual opening of part of thecontainer lid have been made by placing the lid body on a flat lowermold half and pressing a sharp blade having the required contour shapefrom the top surface so that the cutting edge bites into the lid bodywhereby the V-shaped cross-section tear-along groove shown in FIG. 6 isformed and the shape of the opening piece is obtained. With this method,however, since the cutting edge was sharp, the coating film provided inadvance for prevention of corrosion would be broken, necessitatingrepair coating later in the case of a steel lid and thereby resulting inhigher manufacturing costs.

One method which has been recently used to prevent corrosion in place ofthe conventional lacquer coating has been to use a thermoplastic resinfilm at some locations. A thermoplastic resin film has a relativelysuperior drawability, and therefore, possibly would enable processing ofan opening piece without breaking it depending on the processing methodused. When forming a V-shaped cross-section tear-along groove by theabove sharp blade, however, it is not possible to completely avoidbreakage of the resin film.

The easy open lid (or easy open end) used for beverage cans, generalfood cans, and the like may be roughly classified into two types: onesprovided with a tab using the lever principle and ones without a tabwhich allow people to directly push and open the lid by finger pressure.The ones provided with tabs further may be broken down into ones wherethe opening piece comprising part or all of the container lid is tornoff by the tab to separate from the can body, i.e., the tear off types,and ones where it is left attached to the can body, i.e., the stay-ontab types. Both types use a coated aluminum sheet or sheet steel as amaterial, punch them out into the basic lid shape, place them on a flatbottom mold half, press an upper mold half having a sharp edgecross-section scoring blade projecting from it in the shape of thecontour of the opening, and form an opening groove of the shape of theopening in the material. To facilitate the opening of the can, it isnecessary to press the scoring blade to a depth of the opening guidegroove of 1/2 to 2/3 of the thickness of the sheet before processing. Ifthe depth of the opening guide groove is too shallow, the openabilitywill be poor, while if too deep, the strength will be insufficient andthe can will open upon even a small impact from the outside.

When the materials of easy open can lids are made extremely thin due todemands for easy opening etc., a considerable precision has been soughtin the scoring tools as well, resulting in an extremely short life ofthe tools. Due to this problem, as described in Japanese UnexaminedPatent Publication (Kokai) No. 55-70434 and Japanese Unexamined PatentPublication (Kokai) No. 57-175034, measures have been devised forprolonging the tool life by forming a thin upward-facing connectingpiece between the opening piece and its surroundings and then pressingdown the opening piece to make the connecting piece bend in the middleand form the opening guide groove and thereby form a tear-off typeopening piece for the can.

Further, due to the fact that, with the conventional processing of anopening guide groove, defects occur in an organic film layer and themetal surface becomes exposed, repair coating has been applied to ensureresistance to corrosion from the contents and prevent outside rust, butthis repair coating also requires a complicated long baking processsimilar to that of the main coating work and further results in thedischarge of a large amount of solvents contained in the coating, andtherefore, due to environmental considerations, the discharged solventmust be incinerated in a special incinerator. In addition, since theheating during the baking of the coating and the incineration of thesolvent result in the discharge of carbon dioxide, there were concernsof the load on the global environment.

Recently, as proposed in Japanese Unexamined Patent Publication (Kokai)No. 6-115548, Japanese Unexamined Patent Publication (Kokai) No.6-115546, and Japanese Unexamined Patent Publication (Kokai) No.6-122438, to eliminate the need for repair coating and to solve theabove problems, a technique has been developed for producing an easyopen can lid comprising of pressing a plastic laminated metal sheet bythe shoulder radius of an upper and lower mold half to form the openingguide groove, but the openability of the can is not necessarysufficient.

Accordingly, we disclosed, in Japanese Unexamined Patent Publication(Kokai) No. 6-170472, conditions for producing a lid superior inopenability limiting the shoulder radius of the upper and lower moldhalves for forming the tear-along groove and the residual thickness ofthe tear-along groove portion after processing.

Further, the specification alluded to the fact that push-back processingreducing the step difference between the lid body and the openingportion was advantageous to the can openability. Just push-backprocessing alone, however, is not sufficient for achieving an improvedcan openability. In some cases, it causes breakage of the resin film. Ithas become clear that push-back processing suited to the state ofprocessing of the guide groove of the opening portion is necessary. Inparticular, it has become clear that the relationship of the clearancebetween the die and the punch at the time of processing the openingportion, something not alluded to at all in Japanese Unexamined PatentPublication (Kokai) No. 6-170472, is important.

Further, it has become clear that it is possible to improve the canopenability without breaking the surface resin film by the use ofpush-back processing which bends the metal into an S-shape after thepressing.

DISCLOSURE OF INVENTION

Accordingly, an object of the present invention is to eliminate theabove problems in the prior art and to provide the processing conditionsfor an opening portion which maintain the corrosion resistance of thethin resin film bonded to the lid and which enable a good canopenability to be secured.

Another object of the present invention is to further improve the canopenability, that is, while the above pressing and S-shaped push-backprocessing enabled reduction of the thinness and formation of the bentportion without damage to the resin film of the surface of the metalsheet, giving, in the present invention, an S-shaped bent portion withpeak points made the thinnest portions.

In accordance with the present invention, there is provided a processfor producing a resin-coated can lid superior in can openability byprocessing an opening piece of a steel can lid having, on both sides, aresin film having a thickness of 10 to 100 μm and an elongation at breakof at least 100%, comprising the steps of:

pressing the opening piece of the steel can lid such that the radii of apunch and die shoulder are made 0.1 to 1.6 mm, the clearance is made-1.6 to 0.3 mm, and a minimum thickness of a residual thickness of thetear-along groove portion in the opening piece is made of 15 to 100 μm,and then subjecting the opening piece to push-back processing for 0.3 to1.5 times of the step difference amount formed between the lid body andthe opening piece by pressing toward the lid body in the oppositiondirection as the above-mentioned pressing.

In accordance with the present invention, the above push-back processingmay be performed by engaging a bead on the opening piece formed inadvance and a recess portion of the push-back punch to keep the openingpiece from shifting in the horizontal direction with respect to thepunch.

In accordance with the present invention, there is further provided asheet steel easy open can lid laminated with a thermoplastic resinsuperior in can openability formed by pressing and pushing back a sheetsteel having at least on one surface corresponding to the inside surfaceof the can a saturated polyester resin film having a thickness of 10 to100 μm and an elongation at break of at least 100% using the shouldersof upper and lower mold halves for forming tear-along groovesconstituting the shape of the opening piece, having a thinnest portionof the processing having a cross-sectional shape of at least one peakpoint of the push-back processing, and having a thickness of the sheetsteel at the thinnest portion of the processing of 15 to 100 μm.

In accordance with the present invention, there is further provided asheet steel easy open can lid laminated with a thermoplastic resinsuperior in can openability wherein the resin film corresponding to theoutside surface of the can is a polyamide resin having a thickness of 10to 100 μm.

In accordance with the present invention, there is further provided asheet steel easy open can lid laminated with a thermoplastic resinsuperior in can openability wherein the easy open can lid is either ofthe types where the can is opened by a tab separating part or all of thecan lid and where the can is opened by a tab leaving part or all of thecan lid on the can body.

In accordance with the present invention, there is further provided asheet steel easy open can lid laminated with a thermoplastic resinsuperior in can openability wherein the easy open can lid is the typewhere the can is opened without a tab by an opening piece provided atone or more locations of the can lid leaving part of the opening pieceat the can body or separating the opening piece from the can body.

Further, when the characteristic features of the present invention isexplained, the cross-sectional shape of the opening guide grooveobtained by pushing back the pressed portion is made an S-shape and atleast one peak point of the S-shape is made the thinnest portion of theprocessing.

In accordance with the present invention, there is provided a processfor production of a metal easy open can lid superior in can openabilitywherein, when pressing an opening piece of the metal easy open can lid,the pressing is performed such that the thinnest portion in the lateralcross-section of the processed portion displaces to either nonprocessedportion side by 3 to 40% of the length of the thinnest portion in thecross-section starting from the center of the thinnest portion and thenpushback processing is performed so as to form a shape bent at thethinnest portion of processing.

In accordance with the present invention, for the above pressing, thepunch and die shoulders are formed using part of an ellipse having along radius of 0.1 to 5.0 mm and a short radius of 0.05 to 4.0 mm, theangle formed by the long radii of the ellipses of the punch and die isshifted 30 to 150 degrees, and the clearance between the two is made-3.0 to 0.5 times the short radius of the ellipse.

In accordance with the present invention, there is further provided aresin coated easy open can lid superior in can openability, and aprocess for production thereof, comprising, when pressing an openingpiece of a metal easy open can lid having on at least one surfacecorresponding to the inside surface of the can, a coating or a resinfilm having a thickness of 10 to 100 μm and an elongation at break of atleast 100%, pressing and then pushing back so that the thinnest portionin the lateral cross-section of the pressed portion displaces to eithernonprocessed portion side 3 to 40% of the length of the thinnest portionstarting from the center of the thinnest portion in the cross-section soas to form a shape bent at the thinnest portion of the processing.

In accordance with the present invention, for the above-mentionedpressing, the punch and die shoulders are formed using part of anellipse having a long radius of 0.2 to 5.0 mm and a short radius of 0.1to 4.0 mm, the angle formed by the long radii of the ellipses of thepunch and die is shifted 30 to 150 degrees, and the clearance betweenthe punch and the die is made -3.0 to 0 times the short radius of theellipse.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in further detail below withreference to the drawings, wherein:

FIG. 1 is a view of the shape of the tear-along groove between theopening piece and the lid body after the push-back processing;

FIG. 2 is a view of the shape of the opening piece of the lid body afteropening;

FIG. 3(A) and FIG. 3(B) are views of examples of the can lid afterprocessing by the present invention;

FIG. 4 is a view of an example of the pressing;

FIG. 5 is a view of an example of the push-back processing;

FIG. 6 is a view of a tear-along groove of a V-shaped cross-sectionobtained by the method of pressing down a sharp blade;

FIG. 7(A), FIG. 7(B), and FIG. 7(C) are views of easy open can lidsformed by the present invention;

FIG. 8 is a sectional view of a tear-along groove formed by thepressing;

FIG. 9(A) and FIG. 9(B) are sectional views of the tear-along grooveafter the push-back processing of the present invention;

FIG. 10 is an explanatory view of the peak point of the push-backpressing of the present invention;

FIG. 11 is a sectional view of the ideal processing;

FIG. 12 is a view of a model of the residual thickness for the sectionalshape sought after;

FIG. 13 is a view of the pressing mold using the elliptical shape andthe processing shape;

FIG. 14 is a view of the case where the radius of curvature of theellipse is small;

FIG. 15 is a view of the case where the radius of curvature of theellipse is large;

FIG. 16 is a view of the case where the clearance of the upper and lowermold halves is small;

FIG. 17 is a view of the case where the clearance of the upper and lowermold halves is large;

FIG. 18 is a schematic view of an easy open can lid;

FIG. 19 is a view of the sectional shape of an opening guide grooveformed by scoring;

FIG. 20 is a view of a push-back mold chamfered in an arc and theprocessing shape;

FIG. 21 is a view of a model of the residual thickness by the pressing;and

FIG. 22 is a sectional view of the push-back processing on a pressedmaterial.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in further detail below.

The technical concept according to the first aspect of the presentinvention lies in the point of making the residual thickness of thetear-along groove thin without breaking the thin resin film andreprocessing the thinned portion so as to cause embrittlement and toobtain a shape where stress concentration occurs.

Therefore, for the resin for the resin film of the can lid, one is usedwhich has a thickness of 10 to 100 μm, preferably 16 to 80 μm, and anelongation at break of at least 100%, preferably at least 150%. If thethickness is less than 10 μm, the coating will become too thin at thetime of processing and may break, while if over 100 μm, it may not beable to follow the deformation and elongation of the sheet steel at thetime of processing and may peel off. Further, if the elongation at breakis less than 100%, again the coating may not be able to follow thedeformation and elongation of the sheet steel at the time of processingand may break. The reason for making the thickness of the sheet steel atthe thinnest portion 15 of processing after push-back processing 15 to100 μm is that if less than 15 μm, the dropping strength of the easyopen can lid cannot be secured and therefore the lid is not practical,while if over 100 μm, easy openability cannot be secured even at thepeak point 16 of the S-shape of the thinnest portion 15.

As the method for achieving this, first, in the initial processing forforming the tear-along groove, contact of a sharp tool with the resinfilm is avoided and stress concentration is prevented from occurring inthe stress distribution due to the processing. Regarding the former, itis necessary that the radii of the shoulders of the die and punch not bemade too small. Further, if the radii are too large, a tremendous amountof force will be necessary for reducing the thickness of the tear-alonggroove and the risk will rise of the resin film breaking due to thecompressive force. Accordingly, the radius at this portion is made 0.1to 1.6 mm, preferably 0.2 to 1.0 mm. A systematic study was made of themethods for effectively reducing the thickness of the tear-along groovewithout breaking the resin film and, as a result, it was found thatprocessing suitably combining compression, tension, and shearing waseffective. This processing was achieved by limiting the clearancebetween the punch and die to -1.6 to 0.3 mm, preferably -1.0 to 0.0 mm.

If this clearance is less than -1.6 mm, then the compressive stressrequired for reducing the thickness will become too large, while if over0.3 mm, the clearance will become too large and it will not be possibleto reduce the thickness by compression and breakage will occur due tothe tensile force.

On the other hand, regarding the reprocessing of the tear-along groovethinned by the initial processing, it was found that suitable push-backprocessing was effective. Here, the characterizing feature is that bylimiting the above clearance between the punch and die to the optimalconditions, the shape of the tear-along groove shown in FIG. 1 can beobtained after the push-back processing, stress concentration will occurat the tear-along groove portion at the time the can is opened, and thecan will therefore be able to be easily opened. FIG. 2 shows the shapeof the opening portion of the lid body after the can is opened. The tornportion faces downward so that there is almost no possibility of injurywhen one places one's mouth against it. If the amount of the push-backis too small, the necessary can openability cannot be obtained, so it isnecessary that the amount of push-back processing be at least 0.3 timethe step difference caused between the lid body and the opening piecedue to the pressing. However, if too large, the risk rises of theopening portion breaking at the time of reprocessing, so it is made lessthan 1.5 times, preferably 0.6 to 1.3 times. Note that at 1.0 times, thelid body and the opening piece become the same in height. Further, atthe time of push-back processing, the shape of the tear-along groovebecomes stable and forms the shape shown in FIG. 2, so it is preferableto use the lead on the opening piece formed in advance and ensure thatthe scoring does not shift in the horizontal direction with respect tothe punch.

An example of the processing of the can lid according to the firstembodiment of the present invention will be explained below by thedrawings.

FIG. 3(A) and FIG. 3(B) show examples of the can lid after processing bythe present invention. A tear-along groove 3 is formed between the lidbody 1 and the opening piece 2.

FIG. 4 shows an example of the pressing. The resin coated sheet steel 4is sandwiched between one mold half A (punch) 5 and the other mold halfB (die) 6 for the pressing. Here, the step difference t occurs betweenthe lid body 1 and the opening piece 2.

FIG. 5 is an example of the pushback processing. The opening piece 2 ispushed back by the push-back punch 8 from the corresponding side of theinside surface of the can to form the tear-along groove 3.

Here, regarding the type of the resin, basically if the above featuresare satisfied, specifically a polyethylene, polypropylene, polyester,polyamide, an ionomer, etc. may be used alone or in any mixture thereof.The type of the steel of the lid is not particularly limited.

A second embodiment of the present invention will be explained in detailbelow.

The sheet steel used in the present invention is normally one havingmechanical properties of a thickness t₀ of 0.080 to 0.250 mm, a hardness(H_(R30T)) of 46 to 68, and an elongation of about 10 to 60%.

The surface of the sheet steel is plated by one or more metals of Sn,Cr, Ni, Al, or Zn. To eliminate the need for repair coating after thelid-making, a chromate treated coating is laminated with a resin filmsuperior in bonding, processability, and corrosion resistance.

As the sheet steel specifically used, there are a tin-plated sheet steelgiving tin-plating of 0.5 to 3.0 g/m² and then chemical treatment, anickel plated steel sheet giving a nickel plating of 0.3 to 2.0 g/m² andthen chemical treatment, an Sn/Ni plated sheet steel giving a Ni andthen Sn plating of 0.5 to 2.0 g/m² and 0.01 to 0.5 g/m² of Sn and Nideposition, respectively, then anodization, and a chrome-chromatetreated sheet steel normally called TFS (i.e., Tin Free Steel) giving ametal Cr deposition of 50 to 200 mg/m² and a chrome oxide deposition of5 to 30 mg/m².

The laminated resin on at least the can inside surface side of the abovesheet steel is a saturated polyester resin film of a thickness of 10 to100 μm and an elongation of at least 100%. This resin film follows thebase material with good bonding at the time of processing of thetear-along groove by the pressing and is superior in processability as acoating itself, so completely covers the base material at the time ofprocessing and does not require repair coating which had been needed inthe past, and therefore, is an important presence.

The saturated polyester resin in the present invention means a linearthermoplastic polyester obtained by condensation polymerization of adicarboxylic acid and diol and is best represented by polyethyleneterephthalate. As the dicarboxylic acid component, there areterephthalic acid, isophthalic acid, phthalic acid, adipic acid, sebacicacid, azelaic acid, 2,6-naphthalene dicarboxylic acid, decanedicarboxylic acid, dodecane dicarboxylic acid, cyclohexane dicarboxylicacid, and the like alone or in any mixtures thereof. As the diolcomponent, there are ethylene glycol, butadiene diol, decane diol,hexane diol, cyclohexane diol, neopentyl glycol, and the like alone orin any mixtures thereof. Copolymers of two or more dicarboxylic acidcomponents or diol components or copolymers with diethylene glycol,triethylene glycol, and other monomers or polymers are also possible.

Further, the sheet metal resin film used in the present invention mayoptionally have blended therein additives such as plasticizer,antioxidant, thermal stabilizer, inorganic particles, pigment, organiclubricant. Further, when fastening the easy open can lid on a can body,the resin film will sometimes be shaved off and create a problem in workefficiency and appearance. From the viewpoint of the formation of theseam, the resin film on the outside surface is preferably a polyamideresin. The polyamide resin in the present invention means nylon 6, nylon12, nylon 5, nylon 11, nylon 66, etc. alone or in any mixtures thereof.

The thickness of the resin film of the laminated sheet steel used in thepresent invention is made 10 to 100 μm, because, since the sheet steeland the resin film are both formed thinner due to the later mentionedpressing, if less than 10 μm, the barrier property (corrosion resistanceand rust resistance) of the resin film at the processed portion inparticular will not be able to be secured. If over 100 μm, the effectwith respect to the barrier property of the resin film will be saturatedwhich will be disadvantageous economically. When considering thestability of the performance, the economy, etc., a range of 16 to 80 μmis particularly effective. Further, the elongation at break of the resinfilm was limited to one over 100% because if less than 100%, the filmwould break due to the later mentioned pressing or push-back processingand a large number of defects would occur.

Note that the elongation characteristics of the laminated resin filmwere measured by peeling the resin film from the base material thenusing the method of JIS (Japanese Industrial Standard) C 2318.

Next, the processing method and shape will be explained.

In forming the opening portion, with the method of pressing using asharp blade or shearing used in the prior art, repair coating after theshaping and breaking of the resin film becomes necessary so are notpreferable.

As a method of processing for forming a tear-along groove 14 withoutbreaking the sheet steel and the resin film, upper and lower mold halveshaving shoulders having smooth are used, protruding curved surfaces atpositions corresponding substantially to the shape and dimensions of theopening piece 13 and pressing the above-mentioned resin coated sheetsteel. As shown in FIG. 8, a thin portion with a smooth change inthickness is formed, then push-back processing is performed to processthe pressed portion into an S-shape as shown in FIG. 9(A) and FIG. 9(B).The push-back processing gives a thinnest portion 15 formed by thepressing as the peak point 16 of the S-shape. The thickness of the sheetsteel at the processed thinnest portion 4 after the push-back processingis made 15 to 100 μm, but the thickness of the sheet steel can beadjusted mainly by pressing.

In the case of a pushbutton type with no tab, in the push-backprocessing after the pressing, the opening piece portion is processed togive a projecting button shape on the outside surface of the lid.

Here, the present inventors discovered that having the thinnest portion15 positioned at the peak point 16 of the S-shape is an important pointin obtaining an excellent can openability.

The reasons why the can openability can be improved by making the lidthe above shape are believed to be as follows:

In the studies up until now, it was found that the breakage at the timeof opening the easy open can having an opening guide groove of anS-shape formed by pressing and push-back processing occurred startingfrom the peak point portion of the S-shape. This is believed to bebecause the stress at the time of can opening concentrates at the peakpoint portion. Accordingly, by making the peak point portion where thestress when opening the can concentrates and becomes the actual startingpoint of the breakage the thinnest portion of the processing, it isbelieved that the can openability is further improved.

Note that the peak point referred to here will be explained next.

The deepest points of the curved portions formed by the push-backprocessing shown in FIG. 10 will be referred to as the point A and pointB. Next, the position 30% of the length of the line segment A-B from thepoint A or the point B will be referred to as A1 or B1. The vertical tothe line segment A-B passing through the point A1 is referred to as a1and the vertical to the line segment A-B passing through the point B1 isreferred to as b1. The portion receiving the bending at the point A sidefrom the vertical a1 and the portion receiving the bending at the pointB side from the vertical b1 are referred to as the peak points.

The reason why the thickness of the sheet steel of the thinnest portion15 of the processing after the push-back processing was made 15 to 100μm was that if less than 15 μm, the dropping strength of the easy opencan lid cannot be secured and therefore, the lid cannot be practicallyused, while if over 100 μm, can openability cannot be secured even ifthe thinnest portion 15 becomes the peak point 16 of the S-shape. In thecase of a stay-on tab type or partial pull-off type shown in FIG. 7(A),as represented best by the tab type, the thickness of the sheet steel ofthe thinnest portion of processing 15 after the push-back processing ispreferably 35 to 65 μm.

In the case of the pull-off type of pull-open end shown in FIG. 7(B),the can openability is improved from the standpoint of the shape, andtherefore, the thickness of the sheet steel of the thinnest portion ofthe processing 15 after push-back processing is preferably 40 to 100 μm.

Further, compared with the tab type using the lever principle, in thecase of the tab-less type, the thickness of the sheet steel of thethinnest portion of the processing 15 after the push-back processing ispreferably made 25 to 50 μm from the standpoint of the easy openability.

In this series of processing steps, since the resin film having theabove characteristics is elongated evenly together with the basematerial, no processing defects are caused, and therefore, there is noneed for repair coating after the processing and an excellent corrosionresistance can be ensured. Further, according to the process of thepresent invention, since the processing is based on pressing orpush-back processing or other pressing using the shoulder portionshaving mutually projecting smooth curved surfaces, there are almost noneof the problems in tool life seen in the method of pressing a sharpblade, a superior productivity is ensured, and an easy open can lid isobtained.

Below, a detailed explanation will be made of a third embodiment of thepresent invention.

In the present invention, use is made of sheet steel or aluminum sheetor one of the same on whose surface is plated Sn, Ni, Cr, or Zn alone ortogether or one of these given anodization or one of these painted orlaminated with a resin film. The resin film used is one with a thicknessof 10 to 100 μm and an elongation at break of at least 100%. If thethickness is less than 10 μm, the coating becomes too thin at the timeof processing and may break, while if over 100 μm, the coating will notbe able to follow the deformation or elongation of the metal sheet atthe time of processing and may peel off. Further, if the elongation atbreak is less than 100% the coating will again not be able to follow thedeformation or elongation of the metal sheet at the time of processingand may break. An elongation of at least 150% is preferable.

Next, an explanation will be given of the processing shape.

The above-mentioned method of thinning the metal sheet was one ofperforming the pressing setting the upper and lower mold halves,chamfered with the arcs of the radii of curvature R_(P) and R_(D) shownin FIG. 20, giving a certain clearance and pressing the metal sheet bythe shoulder portions. FIG. 21 is a view of a model of the residualthickness of the sectional shape of the metal plate after the pressing.The thinnest portion with the smallest residual thickness is positionedat the center of the section of the processed portion (total length L)which has been thinned. When the push-back processing is then performed,as shown in FIG. 22, the thinnest portion becomes positioned between thetwo upper and lower bent portions resulting from the push-backprocessing.

Therefore, by bending at the thinnest portion with the weakest strengthof the material as shown in FIG. 11, the stress concentration is made toact at the thinnest portion and the can openability can be furtherimproved. To obtain the shape of FIG. 11, it was discovered to make theresidual thickness, which had had the distribution of FIG. 21 in thepast, shift as shown in FIG. 12. That is, by pressing so that thethinnest sectional part displaced from the center of the processedportion (total length L) and then performing the push-back processing,it was discovered that the shape of FIG. 11 could be obtained. Here, theratio of the shift of the thinnest portion from the center of theprocessed portion, that is, the eccentricity ρ={(L/2)-s}/L×100(%), issuitably 3 to 40%. This is because if ρ is less than 3%, there is noeffect of shifting the thinnest portion, while if over 40%, it isdifficult to bend the metal at the thinnest portion and the shape ofFIG. 11 ends up not being able to be obtained.

At this time, the minimum residual thickness after processing in thesheet steel is preferably 15 to 100 μm. This is because if less than 15μm, the dropping strength of the easy open can lid cannot be secured andthe lid cannot be used in practice, while if over 100 μm, the canopenability is inferior. Further, in an aluminum sheet, the minimumresidual thickness is preferably 40 to 200 μm. This is due to similarreasons as the sheet steel, but it is preferable to select the abovevalue in consideration of the difference in material strength.

Next, an explanation will be made of the processing method.

In the pressing using molds chamfered by arcs shown in FIG. 20, thedistribution of the residual thickness becomes symmetrical about theleft and right as shown in FIG. 21. Here, to obtain the distributionwith the eccentric residual thickness as shown in FIG. 12, processing isperformed using part of the ellipse for the shape of the shoulderportion of the pressing molds as shown in FIG. 13. Further, as furthershown in FIG. 13, this is characterized by forming the shoulder portionsof the molds by making the angle formed by the long radial axes of thepunch and the die, that is, φ=30 to 150 degrees. This is because in thecase where the flat directions of the ellipses are the same (0degree≦φ<30 degrees, 150 degrees≦φ<180 degrees), the distributions ofresidual thickness become symmetrical and the eccentric distributionshown in FIG. 12 cannot be obtained. Normally, as shown in FIG. 13, theyare set at φ=90 degrees.

Here, to cause the eccentric distribution of residual thickness shown inFIG. 12, the larger the ratio R_(A) /R_(B) of the long radius and theshort radius the better, but if the value is large, the ellipse becomespointed in shape, so with a resin coated sheet steel, it is believedthat the resin film on the surface of the sheet will be damaged by thesharp end at the time of processing, so 1.5<R_(A) /R_(B) <3 ispreferred.

The dimensions of the ellipse, in the case of a nonlaminated sheetsteel, is a long radius R_(A) of 0.1 to 0.5 mm and a short radius R_(B)of 0.05 to 4.0 mm. This is because if the radius of curvature of themolds is smaller than 0.05 mm, the thinned length in the cross-sectionwill be short, and therefore, it will be difficult to bend the sheet asshown in FIG. 11, so R_(B) ≧0.05 mm (FIG. 14). If the radius ofcurvature of the molds is large, then as shown in FIG. 15 the thinnedlength in the cross-section will become long, thus the pushed backportion will also become long and the stroke of can opening will becomelarge, therefore the long radius of the ellipse R_(A) is made≦5.0 mm.Here, the stroke of can opening means the amount of push-back untilbreakage. The larger the value, the worse the can openability.

In a nonlaminated aluminum sheet, considering the fact that the minimumresidual thickness is greater than that of sheet steel, if the radius ofcurvature of the molds becomes smaller than 0.1 mm, then it becomesdifficult to bend the sheet as shown in FIG. 11, so R_(B) is preferablyat least 0.1 mm. Regarding the upper limit of the radius of the mold, itmay be made the same value as the sheet steel since the residualthickness has no effect. Accordingly, the recommended dimensions of theellipse are a long radius R_(A) of 0.2 to 5.0 mm and a short radiusR_(B) of 0.1 to 4.0 mm.

On the other hand, in a laminated material, the long radius R_(A) ismade 0.2 to 5.0 mm and the short radius R_(B) 0.1 to 4.0 mm. This isbecause with an R_(B) of less than 0.1, the coating on the surface ofthe metal sheet will be damaged, so the short radius R_(B) of theellipse is made at least 0.1. For the long radius, for the same reasonsas a nonlaminated material, R_(A) is made less than 0.5 mm. The sameholds true for sheet steel and aluminum sheet.

By performing the pressing giving a certain clearance CL to the punchand die formed as explained above, a thinnest portion is formed with theeccentric residual thickness shown in FIG. 13. Here, the clearance CL isrepresented by f×R_(B) using the multiple f of the short radius R_(B) ofthe ellipse. For a nonlaminated sheet steel, f is preferably -3.0 to0.3. Here, a negative clearance means the punch and die overlap. When fiis smaller than -3.0, the punch and the die largely overlap and theangle of the portion thinned becomes lateral as shown in FIG. 16, sopush-back processing is not performed well. Further, if f is larger than0.3, the punch and die are separated from each other, so thinningbecomes difficult as shown in FIG. 17.

For a nonlaminated material of aluminum, f is made -3.0 to 0.5. Theupper limit for f is set for the same reason as with sheet steel, but inaluminum's case, considering the fact that the minimum residualthickness is smaller than with sheet steel, f is made less than 0.5.

For laminated material, if the clearance f is made greater than 0, theresin film of the surface of the metal sheet may be damaged at the timeof thinning, so f is made -3.0 to 0. The same is true for sheet steeland aluminum sheet.

After this thinning, the push-back processing is performed from the topand bottom so as to form the pushed back shape bent at the thinnestportion shown in FIG. 11 and form an easy open can lid with good canopenability.

EXAMPLES

The present invention will be explained in further detail in accordancewith examples. The present invention is not of course limited to theseExamples.

Example 1 and Comparative Example 1

Table 1-1 shows the chemical composition of the materials used for thelids. Steel Type 1 is an extremely low carbon steel, Steel Type 2 is alow carbon steel, and Steel Type 3 is a medium carbon steel--allrepresentative types. The thickness used was 0.2 mm. Electro tin platingwas performed to a deposition of 2 to 3 g/m² on the lid use sheet steel,then the tin was made to reflow and then chromate treatment was applied.The sheet steel was again heated and then the resin film shown in Table1-2 was laminated on the two surfaces. Table 1-3 shows the steel types,the composition of the resins, the total elongation, the radius of theshoulders of the die and punch, the amount of clearance between thepunch and the die, the residual thickness of the tear-along groove ofthe opening piece, the amount of push-back, the QTV value, showing theprocess of breaking of the resin film, and the can opening force. Here,the "t" shown in the push-back amount means the step difference of thelid body and opening portion caused at the time of the initialprocessing. Further, the QTV value enables judgement of the films whichwill not break after the lid is immersed in saline containing asurfactant and a voltage of 6 V is applied for 1 minute and a current ofnot more than 1 mA runs. The can opening force is found by measuring themaximum force when opening a stay-on lid type of can. A negative amountof clearance means that the die and the punch partially overlap. Theindication of "x" in the column of the can opening force in the tablemeans that the can could not be opened. The elongation of the resin wasmeasured after peeling it off after laminating.

                  TABLE 1-1    ______________________________________                              (Unit: %)    Steel type           C        Si     Mn   P    S    Al   N    ______________________________________    1      0.0020   0.01   0.16 0.008                                     0.010                                          0.036                                               0.0033    2      0.032    0.02   0.22 0.011                                     0.011                                          0.058                                               0.0023    3      0.123    0.02   0.25 0.012                                     0.008                                          0.062                                               0.0040    ______________________________________

                  TABLE 1-2    ______________________________________                                   Upper and                                          Range of         Upper layer Lower layer   lower  present                 Thick-          Thick-                                       layer  invention         Type of ness            ness  thickness                                              (inside: o,    Code resin   (μm) Type of resin                                 (μm)                                       (μm)                                              outside: x)    ______________________________________    A    Poly-   17      Low melting                                 3     20     o         ester           point                         polyester                         containing                         ionomer    B    Nylon 6 50      Poly-   20    70     o                         propylene                         and                         polyethylene                         copolymer    C    Poly-   30      Low melting                                 20    50     o         ester           point                         polyester    D    Poly-   100     Low melting                                 20    120    x         ester           point                         polyester    E    Poly-   5       Low melting                                 3     8      x         ester           point                         polyester    ______________________________________

                                      TABLE 1-3    __________________________________________________________________________                                 Min. res.                  Radius of                       Radius of thickness                                      Push-  Can Range of          Type             Elongation                  shoulder                       shoulder  of guide                                      back   opening                                                 invention       Steel          of of resin                  of die                       of punch                            Clearance                                 groove                                      am't                                         Q + V                                             force                                                 (inside: ∘,    No.       type          resin             (%)  (mm) (mm) (mm) (μm)                                      (t)                                         (mA)                                             (N) outside: x)                                                      Remarks    __________________________________________________________________________     1 2  C  316  0.5  0.5  -0.5 45   0.95                                         0.2 12  ∘                                                      Inv.     2 2  C  316  0.5  0.5  -1.0 48   0.95                                         0.3 13  ∘                                                      Inv.     3 2  C  316  0.5  0.5  -1.5 52   0.95                                         0.3 13  ∘                                                      Inv.     4 2  C  316  0.5  0.5  -2.0 53   0.95                                         8.6 12  x    Co.     5 2  C  316  0.5  0.5  0    48   0.95                                         0.2 12  ∘                                                      Inv.     6 2  C  316  0.5  0.5  0.2  42   0.95                                         0.3 12  ∘                                                      Inv.     7 2  C  316  0.5  0.5  0.5  x    0.95                                         --  --  x    Co.     8 2  C  316  0.5  0.5  -0.5 68   0.95                                         0.2 17  ∘                                                      Inv.     9 2  C  316  0.5  0.5  -0.5 87   0.95                                         0.3 19  ∘                                                      Inv.    10 2  C  316  0.5  0.5  -0.5 107  0.95                                         0.2 x   x    Co.    11 2  C  316  0.5  0.5  -0.5 46   0.45                                         0.2 17  ∘                                                      Inv.    12 2  C  316  0.5  0.5  -0.5 45   0.20                                         0.4 x   x    Co.    13 2  C  316  0.5  0.5  -0.5 48   1.23                                         0.2 13  ∘                                                      Inv.    14 2  C  316  0.5  0.5  -0.5 52   1.62                                          7.93                                              3  x    Co.    15 2  C  316  0.2  0.2  -0.5 49   0.95                                         0.2 13  ∘                                                      Inv.    16 2  C  316   0.05                        0.05                            -0.5 55   0.95                                         18.2                                             12  x    Co.    17 2  C  316  1.0  1.0  -0.5 52   0.95                                         0.4 14  ∘                                                      Inv.    18 2  C  316  1.5  1.5  -0.5 50   0.95                                         0.3 16  ∘                                                      Inv.    19 2  C  316  2.0  2.0  -0.5 50   0.95                                         6.3 18  x    Co.    20 1  B  220  0.5  0.5  -0.5 49   0.95                                         0.2 11  ∘                                                      Inv.    21 3  A  125  0.5  0.5  -0.5 53   0.95                                         0.3 16  ∘                                                      Inv.    22 2  D  360  0.5  0.5  -0.5 48   0.95                                         0.2 x   x    Co.    23 2  E  275  0.5  0.5  -0.5 50   0.95                                         12.6                                             12  x    Co.    24 2  C  316  0.5  0.5  -0.5 45   0.95                                         0.2 11  ∘                                                      Inv.    __________________________________________________________________________     Note:     "Inv:" indicates present invention, while "Co." indicates comparative     example.

In Experiment Nos. 1, 2, 3, 5, 6, 8, 9, 11, 13, 15, 17, 18, 20, 21, and24 satisfy the present invention, the QTV values were all less than 1mA, there was no breakage of the coatings, and the can opening force wasless than 20 N enabling easy manual opening of the can, it wasconfirmed. Further, in Experiment No. 4 where the clearance was largerthan the range of the present invention in the minus side, thecompressive stress on the resin film was too large and the resin broke.In Experiment No. 7 where the clearance was larger on the plus side,breakage occurred before the thickness of the guide groove was made lessthan 100 μm at the initial processing. In Experiment No. 10, where theminimum thickness of the guide groove was more than 100 μm, with thestay-on type of can opening method, the tab ended up flat and the cancould not be opened. Further, in Experiment No. 12, where the amount ofpush-back was small, the can could similarly not be opened. Conversely,in Experiment No. 14 where the amount of push-back was too large,breakage occured at the guide groove at the time of the push-backprocessing.

In Experiment No. 16, where the radius of the shoulders of the die andpunch was smaller than the range of the present invention, the film wasbroken and a high QTV value was shown. Further, in Experiment No. 19where the radius of the shoulder was conversely too large, a tremendouscompressive stress was required for reducing the residual thickness ofthe guide groove, so the film was damaged. In Experiment No. 22, wherethe resin film was thick, in the stay-on type of can opening method, thetab ended up flat and the can could not be opened. Further, inExperiment No. 23, where the resin film was thinner than the range ofthe present invention, the resin broke at the time of processing theguide groove. Experiment No. 24 was one in which the bead of the openingportion was used to prevent shifting of the opening piece in thehorizontal direction with respect to the punch. In this case, the canopening force can be somewhat lowered since the shape of the guidegroove becomes uniform.

Example 2 and Comparative Example 2

The surface treated sheet steels used are shown in Table 2-1. Thethicknesses and elongations at break of the resin films laminated on thesurface treated sheet steels are shown in Table 2-2 and Table 2-3.Further, the easy open can lids were fabricated by processing of thesurface treated sheet steel based on the above pressing or push-backprocessing. The types of the fabricated easy open can lids, thethicknesses of the thinnest portions after processing, thecross-sectional shapes (FIG. 8 and FIGS. 9(A) and 9(B)), and the resultsof the evaluation are shown in Table 2-2 and Table 2-3.

For the evaluation, the soundness of the coating, the seam property, thecan openability, and the dropping strength were examined.

Note that the soundness of the coating was evaluated by a conductancetest. The seam property was evaluated by visually observing the state offeathering of the outer surface film of the easy open can lid at thetime of fastening the easy open can lid to the can body. The canopenability was judged by whether or not the can could be open fullywithout problem and by measuring the can opening force. The droppingstrength was evaluated by preparing a can with an easy open can lidfilled with some material, dropping it five times vertically from aheight of 60 cm, and examining if the content leaked from the easy opencan lid due to the shock of the drop.

                  TABLE 2-1    ______________________________________                                Apparent weight                                Hydrat-                                      Me-                                ed    tal-                                Cr    lic                  Thick-  Hard- oxide Cr                  ness    ness  *1 (mg/                                      (mg/ Sn    Ni    No.  Name     (mm)    (H.sub.R30T)                                m.sup.2)                                      m.sup.2)                                           (mg/m.sup.2)                                                 (mg/m.sup.2)    ______________________________________    1    TFS      0.255   64    15    100  --    --         chrome-         chromate         treated         sheet steel    2    ET       0.18    59    12    12   1.1   --         electro tin         plated         sheet steel    3    CL Ni    0.20    54    25    15   1050  20         base, thin         Sn plated         sheet steel    4    TFS      0.190   59    15    100  --    --         chrome-         chromate         treated         sheet steel    5    TFS      0.150   54    15    100  --    --         chrome-         chromate         treated         sheet steel    6    ET       0.13    54    12    12   1.1   --         electro tin         plated         sheet steel    7    CL Ni    0.20    51    25    15   1050  20         base, thin         Sn plated         sheet steel    8    TFS      0.100   63    15    100  --    --         chrome-         chromate         treated         sheet steel    ______________________________________     *1 Amount of hydrated Cr oxide shown as amount of Cr.

                                      TABLE 2-2    __________________________________________________________________________           Resin film properties                          Lid shape    Evaluation of performance           Inside surface                  Outside Type                             Thickness     Conduct-           polyester                  surface of of steel  Sound-                                           ance           Film              Elonga-  Film                          easy                             sheet at                                  Push-back                                       ness                                           of           thick-              tion at  thick-                          open                             thinnest                                  sectional                                       of inner                                           inner                                                Can     Drop-       Base           ness              break    ness                          can                             portion                                  shape                                       surface                                           surface                                                open-                                                    Seam-                                                        ping    No.       material           (μm)              (%) Resin film                       (μm)                          lid                             (μm)                                  (FIG.)                                       film                                           (mA) ability                                                    ability                                                        strength    __________________________________________________________________________     1*.sup.1       1   40 229 Nylon 6                       40 FIG.                             55   9A   EG  0.2  EG  EG  EG     2*.sup.1       2   15 170 Nylon 12                       15 7A 75   9A   G   0.8  G   EG  EG     3*.sup.1       3   75 190 Nylon 66                       60 stay                             40   9A   EG  0.1  EG  EG  EG     4*.sup.1       4   40 200 Polyester                       40 on 55   9B   EG  0.2  EG  G   EG     5*.sup.2       1    8 180 Nylon 6                       40 type                             40   9A   P   7.8  EG  EG  EG     6*.sup.2       1   40  90 Nylon 6                       40    110  9A   P   10.5 P   EG  EG     7*.sup.2       1   40 180 Nylon 6                       40    10   9A   G   0.5  EG  EG  P     8*.sup.2       1   40 180 Nylon 6                       40    40   8 .sup.                                       EG  0.2  P   EG  EG     9*.sup.1       1   30 220 Nylon 6                       30 FIG.                             65   9A   EG  0.1  EG  EG  EG    10*.sup.1       2   15 170 Nylon 12                       15 7B 95   9A   G   0.5  G   EG  EG    11*.sup.1       3   75 190 Nylon 66                       45    35   9A   EG  0.2  EG  EG  G    12*.sup.1       4   40 200 Polyester                       40    55   9B   EG  0.2  EG  G   EG    13*.sup.2       1    8 180 Nylon 6                       40    55   9A   P   4.7  EG  EG  EG    14*.sup.2       1   40 185 Nylon 6                       40    110  9A   P   15.5 P   EG  EG    15*.sup.2       1   40 180 Polyester                       40    10   9A   G   0.8  EG  EG  P    16*.sup.2       1   40 180 Nylon 6                       40    55   8 .sup.                                       EG  0.3  P   EG  EG    __________________________________________________________________________     *.sup.1 : Example     *.sup.2 : Comparative Example     Note)     Underlines indicate out of invention.     Evaluation of performance:     EG: extremely good,     G: good,     P: poor (not practical).

                                      TABLE 2-3    __________________________________________________________________________           Resin film properties                          Lid shape    Evaluation of performance           Inside surface                  Outside Type                             Thickness     Conduct-           polyester                  surface of of steel  Sound-                                           ance           Film              Elonga-  Film                          easy                             sheet at                                  Push-back                                       ness                                           of           thick-              tion at  thick-                          open                             thinnest                                  sectional                                       of inner                                           inner                                                Can     Drop-       Base           ness              break    ness                          can                             portion                                  shape                                       surface                                           surface                                                open-                                                    Seam-                                                        ping    No.       material           (μm)              (%) Resin film                       (μm)                          lid                             (μm)                                  (FIG.)                                       film                                           (mA) ability                                                    ability                                                        strength    __________________________________________________________________________    17*.sup.1       5   40 220 Nylon 6                       40 FIG.                             40   9A   EG  0.2  EG  EG  EG    18*.sup.1       6   15 170 Nylon 12                       16 7C 35   9A   G   0.3  EG  EG  EG    19*.sup.1       7   75 190 Nylon 66                       60    20   9A   EG  0.1  EG  EG  G    20*.sup.1       8   40 200 Polyester                       40    45   9B   EG  0.2  EG  G   EG    21*.sup.2       5    8 180 Nylon 6                       40    40   9A   P   7.3  EG  EG  EG    22*.sup.2       5   40  90 Nylon 6                       40    110  9A   P   10.5 P   EG  EG    23*.sup.2       5   40 180 Nylon 6                       40 FIG.                             10   9A   G   0.8  EG  EG  P                          7B    24*.sup.2       5   40 180 Nylon 6                       40 C  40   8 .sup.                                       EG  0.2  P   EG  EG    __________________________________________________________________________     *.sup.1 : Example     *.sup.2 : Comparative Example     Note)     Underlines indicate out of invention.     Evaluation of performance:     EG: extremely good,     G: good,     P: poor (not practical).

Example 3 and Comparative Example 3

The specifications of the metal sheets used are shown in Table 3-1 andTable 3-3.

The evaluations of the eccentricity of the easy open can lids, theminimum thicknesses, the can openability, and the soundness of the filmsformed by the thinning and then push-back processing changing theconditions of the pressing for nonlaminated and laminated materials areshown in Table 3-2 and Table 3-4. The can openability is evaluated fromthe push-down can opening load and amount of push in up to when the canis opened. The soundness of the film is evaluated by the lack ofbreakage of the film when a current of less than 1 mA is passed bypassing a current to the resin film.

                  TABLE 3-1    ______________________________________    Specifications of Laminated Sheet Steel    Used in Experiments (Nonlaminated Materials Only    Underlying Base Materials)                                       Thickness                                       before            Name          Material     processing    ______________________________________    Laminated            Upper layer resin film                          PET          40 μm    sheet steel            Base material Tin free steel T-2.5                                       190 μm            Lower layer resin film                          PET          40 μm    ______________________________________

                                      TABLE 3-2    __________________________________________________________________________    Experimental Findings (Base Material: Sheet Steel)                                       Min.                        Angle φ of residual    Presence            Shape of mold                        upper and lower                                Eccentricity ρ                                       thickness                                            Can   Film    No.       of film            R.sub.A               R.sub.B                  f*.sup.1                     CL*.sup.2                        mold halves                                (%)    (μm)                                            openability                                                  soundness                                                       Remark    __________________________________________________________________________     1 Nonlam.            1.0               0.5                  -0.1                     -0.05                        90       6     45   G     --   Inv.     2 Laminated            1.5               0.5                  -0.8                     -0.4                        90      35     55   G     G    Inv.     3 Laminated            1.0               0.5                  -1 -0.5                        90      15     75   G     EG   Inv.     4 Nonlam.            1.0               0.5                  -0.8                     -0.4                        45      12     47   G     --   Inv.     5 Laminated            1.0               0.5                  -1 -0.5                        120     15     51   G     EG   Inv.     6 Nonlam.            0.2               0.1                  -1 -0.5                        90      13     45   EG    --   Inv.     7 Laminated            0.7               0.3                  -1 -0.3                        90      14     56   EG    G    Inv.     8 Laminated            1.0               0.5                  -1 -0.5                        90      15     40   EG    EG   Inv.     9 Nonlam.            1.2               0.5                  -0.8                     -0.4                        90      25     62   EG    --   Inv.    10 Nonlam.            3.0               1.5                  -1 -1.5                        90      18     56   G     --   Inv.    11 Laminated            0.4               0.2                  -1 -1.0                        90      14     52   EG    G    Inv.    12 Nonlam.            1.2               0.5                  -1.2                     -0.7                        90      12     61   EG    --   Inv.    13 Nonlam.            1.0               0.5                  0  0  90       8     45   G     --   Inv.    14 Laminated            1.0               0.5                  -0.3                     -0.15                        90      10     54   G     G    Inv.    15 Laminated            1.0               0.5                  -2.5                     -0.75                        90       5     56   G     G    Inv.    16 Laminated            2.0               0.5                  -0.8                     -0.4                        90      45     57   P     G    Comp.    17 Nonlam.            1.0               0.5                  -1 -0.5                        90      11     90   P     --   Comp.    18 Nonlam.             0.02                0.01                  -1 -0.01                        90      14     46   P     --   Comp.    19 Nonlam.            6.0               3.0                  -1 -3.0                        90      22     45   P     --   Comp.    20 Laminated            0.1                0.05                  -1 -0.05                        90      14     56   G     P    Comp.    21 Laminated            6.0               3.0                  -1 -3.0                        90      22     51   P     P    Comp.    22 Nonlam.            1.0               0.5                  0.7                     0.35                        90       0     (20) P     --   Comp.    23 Laminated            1.6               0.8                  0.4                     0.32                        90       3     41   P     P    Comp.    24 Laminated            1.0               0.5                  -3.5                     -1.75                        90      --     45   P     P    Comp.    __________________________________________________________________________     *.sup.1,.sup.2 : Provided, however, that clearance is expressed by f, CL     using CL = f × R.sub.8.     Evaluations:     EG: extremely good,     G: good,     P: poor (not practical).

                  TABLE 3-3    ______________________________________    Specifications of Laminated Aluminum Sheet    Used in Experiments (Nonlaminated Materials Only    Underlying Base Materials)                                    Thickness before            Name           Material processing    ______________________________________    Laminated            Upper layer resin film                           PET      40 μm    sheet   Base material  5052 H38 250 μm            Lower layer resin film                           PET      40 μm    ______________________________________

                                      TABLE 3-4    __________________________________________________________________________    Experimental Findings (Base Material: Aluminum Sheet)                                       Min.                        Angle φ of residual    Presence            Shape of mold                        upper and lower                                Eccentricity ρ                                       thickness                                            Can   Film    No.       of film            R.sub.A               R.sub.B                  f*.sup.1                     CL*.sup.2                        mold halves                                (%)    (μm)                                            openability                                                  soundness                                                       Remark    __________________________________________________________________________     1 Nonlam.            1.0               0.5                  -0.1                     -0.05                        90       6      96  G     --   Inv.     2 Laminated            1.5               0.5                  -0.8                     -0.4                        90      35     116  G     G    Inv.     3 Laminated            1.0               0.5                  -1 -0.5                        90      15     179  G     EG   Inv.     4 Laminated            1.0               0.5                  -1 -0.5                        120     15     124  G     EG   GInv.     5 Nonlam.            0.2               0.1                  -1 -0.5                        90      13     137  EG    --   Inv.     6 Laminated            0.7               0.3                  -1 -0.3                        90      14     110  EG    G    Inv.     7 Laminated            1.0               0.5                  -1 -0.5                        90      15     121  EG    EG   Inv.     8 Nonlam.            1.2               0.5                  -0.8                     -0.4                        90      25      89  EG    --   Inv.     9 Nonlam.            3.0               1.5                  -1 -1.5                        90      18     108  G     --   Inv.    10 Laminated            0.4               0.2                  -1 -1.0                        90      14     128  EG    G    Inv.    11 Nonlam.            1.0               0.5                  0  0  90       8      97  G     --   Inv.    12 Laminated            1.0               0.5                  -0.3                     -0.15                        90      10     102  G     G    Inv.    13 Laminated            1.0               0.5                  -2.5                     -0.75                        90       5     129  G     G    Inv.    14 Laminated            2.0               0.5                  -0.8                     -0.4                        90      45     156  P     G    Comp.    15 Nonlam.            1.0               0.5                  -1 -0.5                        90      11     237  P     --   Comp.    16 Nonlam.            0.1                0.05                  -1 -0.01                        90      14     142  P     --   Comp.    17 Nonlam.            6.0               3.0                  -1 -3.0                        90      22     125  P     -    Comp.    18 Laminated            0.1                0.05                  -1 -0.05                        90      14     113  G     P    Comp.    19 Laminated            6.0               3.0                  -1 -3.0                        90      22     120  P     P    Comp.    20 Nonlam.            1.0               0.5                  1.0                     0.5                        90       0     (190)                                            P     --   Comp.    21 Laminated            1.6               0.8                  0.4                     0.32                        90       3     102  P     P    Comp.    22 Laminated            1.0               0.5                  -3.5                     -1.75                        90      --     122  P     P    Comp.    __________________________________________________________________________     *.sup.1,.sup.2 : Provided, however, that clearance is expressed by f, CL     using CL = f × R.sub.8.     Evaluations:     EG: extremely good,     G: good,     P: poor (not practical).

INDUSTRIAL APPLICABILITY

According to the present invention, it becomes possible to improve theshort life of scoring blades which had been a problem in theconventional scoring. Further, by using a thermoplastic resin laminatedmetal sheet as the material, it becomes possible to eliminate damage tothe resin film on the surface and obtain a metal easy open can lid notrequiring repair coating after processing. Further, by using athermoplastic resin laminated sheet steel, use of steel for the easyopen can lid becomes possible and use of just steel for the can can berealized and therefore low cost production of a product suited forrecycling to alleviate the environmental load can be realized.

We claim:
 1. A process for producing a resin coated can lid having anopening piece comprising:providing a steel can lid body having a resinfilm on both sides, said resin film having a thickness of 10 to 100 μmand an elongation at breakage of at least 100%; providing a punch anddie for pressing said lid body for forming an opening piece, said punchand die each having a pressing shoulder, each pressing shoulder having aradius of 0.1 to 1.6 mm, with clearance between the pressing shoulder ofthe punch and the pressing shoulder of the die being -1.6 to 0.3 mm;pressing said lid body with said punch and die to provide said openingpiece displaced at a step difference from said lid body, with a residualtear-along groove portion disposed between said lid body and saidopening piece, said tear-along groove portion having a minimum thicknessof 15 to 100 μm; push-back processing of said opening piece toward saidlid body by displacing said opening piece a distance of 0.3 to 1.5 timessaid step difference formed by said pressing in a direction opposite tosaid pressing.
 2. A process according to claim 1 furthercomprising:forming a bead on said opening piece during formation of saidopening piece in said pressing step; providing a push-back punch havinga recess portion for engaging said bead of said opening piece; engagingsaid bead of said opening piece with said recess of said push-back punchduring push-back processing for preventing horizontal movement of saidopening piece during push-back processing.
 3. A steel sheet easy opencan lid formed by a pressing and a push-back processing methodcomprising:a can lid body including an opening piece and a tear-alonggroove, said tear-along groove being disposed between the can lid bodyand the opening piece, said can lid body, said opening piece, and saidtear-along groove being formed from a single piece of steel sheet; aside of said can lid body, said opening piece and said tear-along groovecorresponding to an inside surface of the can has laminated thereon asaturated polyester resin film having a thickness of 10 to 100 μm and anelongation of at least 100% at breakage; said tear-along groove having acompressed S-shape cross-section having a thinnest thickness of 15 to100 μm located at a deepest point of curvature of one curve of theS-shaped cross-section.
 4. A steel sheet easy open can lid according toclaim 3, wherein a side of the can lid body, the opening piece, and thetear-along groove corresponding to an outside surface of the can haslaminated thereon a polyamide resin having a thickness of 10 to 100 μm.5. A steel sheet easy open can lid according to claim 3 furthercomprising:said opening piece has attached thereto a tab for removingsaid opening piece from said can lid; and said opening piece comprisesone of a portion of the can lid and substantially all of the can lid. 6.A steel sheet easy open can lid according to claim 3 furthercomprising:said opening piece does not have attached thereto a tab forremoving said opening piece from said can lid; and said opening piececomprises one of a portion of the can lid and substantially all of thecan lid.
 7. A process for producing a metal sheet easy open can lidcomprising:providing a piece of metal sheet for forming the metal sheeteasy open can lid; pressing the piece of metal sheet between a punch anda die forming a thinned processed portion disposed between an unthinnednon-processed portion corresponding to a can lid body and an unthinnednon-processed portion corresponding to an opening piece; said thinnedprocessed portion having a length with a center; said pressing resultingin a thinnest cross-sectional area of said thinned processed portionbeing displaced 3 to 40% of the length of said thinned processed portionfrom the center of the length of said thinned processed portion; forminga tear-along groove between said can lid body non-processed portion andsaid opening piece non-processed portion by push-back processing in adirection opposite to pressing to bend said thinned processed portion atsaid displaced thinnest cross-sectional area.
 8. A process for producinga metal sheet easy open can lid according to claim 7 furthercomprising:providing said punch with a shoulder and providing said diewith a shoulder, with the shoulder of each of the punch and the diehaving a form of part of a ellipse having a long radius of 0.1 to 5.0 mmand a short radius of 0.05 to 4.0 mm, with an angle of 30 to 150 degreesformed by the long radius of the ellipse of each of the punch and thedie shoulders; and providing a clearance between the punch shoulder andthe die shoulder of -3.0 to 0.5 times the short radius of the ellipse.9. A process for producing a metal sheet easy open can lidcomprising:providing a piece of metal sheet for forming the metal sheeteasy open can lid, said piece of metal having on at least a sidecorresponding to an inside surface of the can at least one of a coatingand a resin film having a thickness of 10 to 100 μm and a elongation atbreakage of at least 100%; pressing the piece of metal sheet between apunch and a die forming a thinned processed portion disposed between anunthinned non-processed portion corresponding to a can lid body and anunthinned non-processed portion corresponding to an opening piece; saidthinned processed portion having a length with a center; said pressingresulting in a thinnest cross-sectional area of said thinned processedportion being displaced 3 to 40% of the length of said thinned processedportion from the center of the length of said thinned processed portion;forming a tear-along groove between said can lid body non-processedportion and said opening piece non-processed portion by push-backprocessing in a direction opposite to pressing to bend said thinnedprocessed portion at said displaced thinnest cross-sectional area.
 10. Aprocess for producing a metal sheet easy open can lid according to claim9 further comprising:providing said punch with a shoulder and providingsaid die with a shoulder, with the shoulder of each of the punch and thedie having a form of part of an ellipse having a long radius of 0.2 to5.0 mm and a short radius of 0.1 to 4.0 mm, with an angle of 30 to 150degrees formed by the long radius of the ellipse of each of the punchand the die shoulders; and providing a clearance between the punchshoulder and the die shoulder of -3.0 to 0 times the short radius of theellipse.
 11. A metal sheet easy open can lid formed from a pressing andpush-back processing method comprising:a can lid body, an opening piece,and a tear-along groove, said tear-along groove being disposed betweenthe can lid body and the opening piece; the can lid body, the openingpiece, and the tear-along groove all being formed from a single piece ofmetal sheet; at least one of a coating and a resin film having athickness of 10 μm to 100 μm and an elongation at breakage of at least100% disposed on at least a side of said can lid body, said openingpiece and said tear-along groove corresponding to an inside surface ofthe can; said tear-along groove having a compressed S-shapedcross-section; said tear-along groove being a pressed processed portionof said single piece of metal sheet and said can lid body and saidopening piece being a non-pressed process portion of said single pieceof said metal sheet, with said tear-along groove pressed process portionhaving a cross-sectional thickness thinner than a cross-sectionalthickness of said non-pressed process portions of said can lid body andsaid opening piece; said tear-along groove pressed process portionhaving a thinnest cross-sectional thickness at a distance of 3 to 40% ofa length of the pressed process portion displaced from a center of thelength of the pressed process portion prior to bending, with saidthinnest cross-sectional thickness located at one bend of saidcompressed S-shape cross-section after bending into said compressedS-shaped cross-section of said tear-along groove.